1. Field of the Invention
This invention relates generally to an apparatus for cutting relatively short lengths of tubing from a longer tube, and more specifically, this invention relates to a tube cutting apparatus of the "shimmy die" type.
2. Description of the Prior Art
Over the years there have been many attempts to produce machines that could cut relatively long lengths of tube into desired shorter tubing lengths. For example, there has been the "cutting roll" or "pinch roll" approach exemplified by the Bertoletti device, which operated on the same principle as the pipe cutters utilized by plumbers. Another effort involved the "screw" machine, in which a stationary knife bears against a tube that is rotated. Both of these devices have the disadvantage that a burr or rough edge is formed along the inner periphery of the cut. Obviously, there are many applications in which such a burr cannot be tolerated, while in many other applications it at least renders the tubing lengths undesirable. Later attempts to improve these lengths of tubing involved grinding off the burr, which of course involves a completely separate step to remove the burr on each of the cut length of tubing. Such an approach is not only time consuming, but also very expensive, and the resulting cut surfaces are still frequently not as smooth as desired.
An improvement over the "screw" machine is one exemplified by the Continental apparatus, in which a mandrel is located down the center of the tubing being cut. This means that the cutting is actually produced by a pinching action between the stationary knife and the inner mandrel. While somewhat alleviating the burr problem on the inside edge, however this approach does not completely eliminate the burr. In addition, the length of the tubing cut varies with the sharpness of the knife or cutter and a jagged cut results.
Still another improvement over the basic "screw" machine is one exemplified by the Burr Oak apparatus, in which the tubing is maintained stationary and the knives are rotated to cut the tube. This approach is also quite expensive, and the inner burr is not eliminated.
The most desirable prior art tube cutting apparatus is that utilizing the "shimmy die" approach. The basic "shimmy die" tube cutting apparatus is disclosed in U.S. Pat. No. 2,627,921, issued to Charles M. Brehm on Feb. 10, 1953. In this type of machine, the cutting action is produced by a pair of cutting dies about the outer periphery of the tubing, the cutting dies being moved eccentrically with respect to each other. A pair of punches are located within the tube and are also adapted to move radially and rotationally with respect to each other, in order to follow the dies during the cutting action. In this fashion, a very clean cut, with minimum burrs, can be achieved.
The Brehm shimmy die machine is now produced by Vulcan Tool Company. In the Valcum Tool version of the Brehm shimmy die machine, a long mandrel is located in and extends along the entire length of the tube to be cut. The mandrel is connected to one of the punches, and a flexible tension rod passes through and interconnects and punches. The punches are securely fashioned to the tension rod, so that the punches may be accurately positioned with respect to the outer cutting dies. Of course, the tension rod that passes through the punches has to be sufficiently flexible to permit the punches to move with respect to each other during the "shimmy" cutting operation. The tube to be cut is fed over the punches and the mandrel for its entire length, and then backed out during the cutting operation. While this Brehm shimmy die machine provides a much better tube cutting action than any previous device, it still is plagued by a number of problems and deficiencies.
One of the obvious difficulties is that due to the front loading (i.e., the insertion or feeding of the tube to be cut over the punches and mandrel from the front), the length of the tube being cut must be accommodated not only on the side of the machine where it passes over the mandrel, but also on the side of the machine from which it is fed. As these tube lengths are often twenty feet, or even greater, this means that over forty feet of space must be provided for the machine. Also, as a consequence of this front-loading procedure, the tube cutting machine cannot be run continuously but must remain inactive while a new length of tube is being fed onto the mandrel. Further, a related problem is that only straight tube can be cut. Since much tubing is provided in a coiled form, this means that the coils must be straightened and cut into manageable lengths before being fed into the machine, with the consequent loss of time and increase in expense.
The long mandrel required in the Brehm shimmy die machine also creates a number of difficulties. It is obvious that when the punches have to be accurately positioned within a tenth of a thousandth of an inch, for the best results, and they are located on an end of a mandrel that is twenty feet or more in length, there are substantial difficulties in positioning the punches with the proper degree of accuracy. This means that when the dies and punches are being changed for a different size of tubing, there is much delay and effort involved in preparing the machine for operation. Also, with the long length of the mandrel, temperature variations are magnified and the tight tolerances of the punches can be adversely affected by even a few degrees change in temperature. This not only relates to ambient temperature changes, but the temperature of the tube stock itself.
Other difficulties are encountered as a result of the interconnection of the punches by the tension rod passing therethrough. For one thing, the construction of the punches with the opening through them for the tension rod makes their production, and hence replacement, cost relatively quite high. Since a different set of punches are needed for each tubing size that is to be cut, this can greatly add to the expense of the machine. In addition, the necessity of passing the tension rod through the punches limits the minimum size of the punches without detracting from the support that they must provide during the cutting operation. Hence, the minimum size tube that can be cut is also limited. Further, the requirement of flexibility of the tension rod passing through the punches means that this member must be relatively small. Consequently, this tension rod is relatively weak and much breakage is encountered.
Other disadvantages of the Brehm shimmy die machine involve the relatively high overall cost of the machine, as well as the necessity for a relatively expensive feed apparatus.